This disclosure relates to magnetorheological fluid compositions, and more particularly to high yield stress magnetorheological (MR) fluid compositions. The high yield stress fluid compositions include high aspect ratio magnetizable particles and unimodal low aspect ratio magnetizable particles in a carrier fluid.
Fluid compositions that undergo a change in apparent viscosity in the presence of a magnetic field are referred to as Bingham magnetic fluids or magnetorheological fluids. Magnetorheological fluids generally include magnetizable particles dispersed or suspended in a carrier fluid. In the presence of a magnetic field, the magnetizable particles become polarized and are thereby organized into chains of particles or particle fibrils within the carrier fluid. The chains of particles act to increase the apparent viscosity or flow resistance of the fluid composition resulting in the development of a solid mass having a yield stress that must be exceeded to induce onset of flow of the magnetorheological fluid. When the flow of the fluid composition is restricted as a result of orientation of the particles into chains, the fluid composition is said to be in its “on state”. The force required to exceed the yield stress is referred to as the “yield strength”. In the absence of a magnetic field, the particles return to an unorganized or free state and the apparent viscosity or flow resistance of the fluid composition is then correspondingly reduced. The state occupied by the composition in the absence of a magnetic field is referred to as the “off-state”.
Commonly used magnetorheological fluids generally employ magnetizable particles that are symmetrical and have aspect ratios of about 1 to about 1.5. Examples of such particles are spherical particles, ellipsoids, cuboids, or the like. Magnetorheological fluids employing the aforementioned particles are used in devices or systems such as clutches, dampers, actuators, and the like.
In a magnetorheological device, it is often desirable to maximize the ratio of the on-state force to the off-state force in order to maximize the controllability offered by the device. Since the on-state force is dependent upon the magnitude of the applied magnetic field, the on-state force should remain constant at any given applied magnetic field. If the off-state force increases over time because the off-state viscosity is increasing but the on-state force remains constant, the on-state/off-state ratio will decrease. This decrease in the on-state/off-state ratio results in undesirable minimization of the controllability offered by the device. A more durable magnetorheological fluid that does not thicken over an extended period of time, preferably over the life of the device would be very useful.